How is fluorescence different from phosphorescence?

Fluorescence and phosphorescence are both radiative processes by which an excited molecule returns to its ground state, but they differ primarily in their electronic mechanisms and the duration of light emission.1

• Electronic Transitions: In organic molecules, fluorescence involves transitions between electronic states of the same multiplicity (typically singlet to singlet). In contrast, phosphorescence involves transitions between levels of different multiplicity (typically triplet to singlet). Because a change in electron spin is quantum mechanically "forbidden" or highly improbable, these transitions are slow.213
• Persistence: Fluorescence is an almost instantaneous effect, typically ending within approximately ${10}^{-8}$ seconds after excitation ceases. Phosphorescence is characterized by a persistent afterglow that can last from milliseconds to years, as the electron is often trapped in a metastable state before returning to the ground level.231
• Energy: Because triplet states are generally lower in energy than the corresponding excited singlet states, phosphorescence typically occurs at longer wavelengths than fluorescence.1

Both processes are subject to nonradiative competition, such as internal conversion or intersystem crossing, which reduces the overall quantum yield of the light emission.1